1. Field of the Invention
Coal is an important fuel for the generation of power and its continued use for this purpose depends in part on the inexpensive availability of coal at the various power plants located throughout the country. A determining factor of the cost of the coal at the site of the power plants is the cost and equipment needed to deliver the coal to the site.
In the past, various types of delivery systems have been devised to maximize the economic advantages of coal usage. Rail, truck and barge delivery have been in use for many years but these modes of transportation have been recently giving way to more economical transport by means of pipeline. One such pipeline transportation method utilizes coal in the form of a coal/water slurry which is pumped through the pipes to the point of usage, i.e., power plant. While numerous economic and environmental advantages have been realized by coal/water slurry pipeline transportation, they all suffer from the fact that the presence of water, which cannot all be easily removed, materially decreases the overall Btu content of the coal since the substantial portion of its heating value must be expended to vaporize the water.
More recently it has been proposed to transport finely pulverized coal in a coal/liquid CO.sub.2 slurry. This technique has offered certain benefits over the coal/water slurry method since it lacks the presence of energy wasting water. In addition, the use of liquid CO.sub.2 made the slurry easier to pump and hence lowered the delivery costs. However, the ease of pumping was only achieved by keeping the particle size of the coal very small. For example, in U.S. Pat. Nos. 4,206,610 and 4,377,356, the particle size had to be such that they would pass through a U.S. 50 mesh, i.e., the particles had to be no greater than 300 microns in diameter.
The small particle size limitation requires specialized equipment to pulverize the coal at the site of mining as well as specialized equipment to separate the small particles from the liquid CO.sub.2 at the site of use. For this reason the fine particle size slurry method does not lend itself for easy integration with existing wash plants, mines and boilers. Also, additional energy is needed to pulverize the coal to the necessary small particle size. Furthermore, the use of a fine particle size slurry does not easily lend itself to optional transportation by rail truck or barge. Thus, less options are easily available when using the fine particle size slurry which can be disadvantageous in the event that there is a pipeline failure.
Attempts have been made in the past to avoid the problems associated with small particle size slurries by using large particles of coal. However, these prior attempts to use pipeline transportation technology to transport large or coarse particles of coal in liquid CO.sub.2 have not been met with success. The movement of larger more coarse particles of coal usually requires higher velocities in the pipeline than that required for small particle size slurry transportation.
These higher velocities greatly increase the amount of energy needed for pumping and considerably increase the wear in the pipeline which occurs at the higher velocities. Therefore, there still exists the need in the art to expeditiously transport coal by pipeline which utilizes large particles of coal so as to avoid the problems associated with small particle size slurries and which also avoids the large energy requirement and wear associated with present large particle size pipeline transporation techniques.